Optical disc having a protective layer which is thicker at its outer periphery

ABSTRACT

An optical disc having a functional film, formed by a thin metal plate, and a protective film, formed on a transparent substrate, is disclosed. The protective film satisfies a condition d≧2d 0 , where d is a maximum film thickness at an outer periphery of the protective film and d 0  is a mean film thickness at a flat portion on a data area. A magneto-optical disc having a functional film which is a metal thin film having magneto-optical effects is also disclosed, wherein the protective film satisfies a condition that a difference L 1  between a maximum film thickness d at the outer periphery of the protective film and a mean film thickness d 0  at a flat portion of the data area is 40 μm or less and the protective film has a curved surface having a radius of curvature R 1  ≧12.48 mm at the outer periphery of the protective film. The present invention provides an optical disc having superior recording properties and a sufficient corrosion resistance even when the disc is provided with a highly corrosive thin metal film, such as a rare earth-transition metal amorphous thin film.

This is a divisional of application Ser. No. 08/908,739, filed Aug. 7,1997, which is a continuation of Ser. No. 07/994,662 filed Dec. 22, 1992now U.S. Pat. No. 5,916,638.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to an optical disc and, more particularly, toimprovement of a protective film on the optical disc and of a magnetichead device employed with the optical disc.

The optical disc for recording and/or reproducing information by laserlight radiation, the digital audio disc or compact disc and the opticalvideo disc or laser disc, have become popular.

The optical disc, such as a digital audio disc, has a transparentsubstrate, a pattern of pits and lands corresponding to informationsignals, and a reflective film formed by a thin metal film, such as anAl film, and a protective film for separating the reflective film frommoisture in air and oxygen, are formed in this order on the substrate.The protective film is a layer of UV curable resin which is usuallyapplied uniformly on the entire surface of the reflective film by spincoating or roll coating and cured in situ.

Meanwhile, with the above-described optical disc, only the informationsignals formed by the manufacturer on the transparent substrate areread, while it is not possible for the user to write information signalsdirectly on the optical disc. Recently, a magneto-optical disc has beendeveloped as an optical disc on which information signals can berecorded and erased repeatedly by the user, and presented to the market.

With the above-described magneto-optical disc, a magnetic thin filmhaving an axis of easy magnetization in a direction perpendicular to thefilm surface and exhibiting large magneto-optical effects is employed.For recording, the magnetic thin film is partially raised intemperature, by radiating a laser light beam, to a temperature higherthan the Curie temperature or a compensation temperature to extinguishcoercivity at the heated area for aligning the direction ofmagnetization with that of an external recording magnetic field. Forplayback, a laser light is radiated on the thin magnetic film to readout the recorded information by taking advantage of the magnetic Kerreffect or the Faraday effect.

The thin magnetic film having properties required of the recordingmagnetic layer may be exemplified by a rare earth-transition metalamorphous thin film, such as a TbFeCo based amorphous thin film.

The magneto-optical disc has a recording section formed by stacking areflective layer and a dielectric layer on the rare earth-transitionmetal amorphous thin film, and the protective film is formed on therecording section. The magneto-optical discs are of two types, that isof a single side type and a double side type in which twomagneto-optical discs are bonded together with the protective filmsfacing each other so that recording/playback may be made from bothsides. The single side type magneto-optical disc is attracting attentionas a standard item.

Meanwhile, since the rare earth-transition metal amorphous thin film,formed as a recording magnetic layer, is highly corrosive, awater-proofness higher than that demanded of an optical disc, such as adigital audio disc, is demanded of the protective film of themagneto-optical disc. Above all, for the single side typemagneto-optical disc, the demand laid on its protective film is moresevere because the protective film is exposed directly to atmosphere.

For this reason, attempts have been made for forming a UV curable resinlayer of a larger thickness or a higher cross-linking degree on therecording section as a protective layer for a magneto-optical disc.However, if the UV curable resin layer of a larger film thickness or ahigher cross-linking degree is formed on the recording section, thesubstrate tends to be warped due to curing contraction. Consequently, aUV curable resin layer having the same film thickness and cross-linkingdegree as that of the digital audio disc is formed as a protective film,for the magneto-optical disk and sufficient resistance to corrosion isnot achieved.

OBJECT AND SUMMARY OF THE INVENTION

In view of the above-described status of the art, it is therefore anobject of the present invention to provide an optical disc in whichsufficient corrosion resistance may be achieved when it has a highlycorrosive functional film, such as a rare earth-transition metalamorphous thin film.

According to the present invention, there is provided an optical dischaving a functional film, formed by a thin metal film, and a protectivefilm, characterized in that a maximum film thickness d at an outerperiphery of said protective film and a mean film thickness d₀ at a flatportion on a data area of the disc are related by the formula d≧2.0 d₀.

According to the present invention, sufficient corrosion resistance maybe realized even when a highly corrosive metal thin film, such as a rareearth-transition metal amorphous thin film, is formed as a functionalfilm, and satisfactory recording characteristics may be achieved.Preferably, a radius of curvature of the protective film at the outerperiphery of the film is equal to or greater than a radius of curvatureof an outer portion of a sliding contact surface of a magnetic headdevice, and the radius of curvature of the protective film at the outerperiphery is equal to or greater than 12.48 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a surface shape profile of an optical discaccording to the present invention.

FIG. 2 is a graph showing a surface shape profile of a conventionaloptical-disc.

FIG. 3 is a side view showing an example of a magnetic head deviceadapted for the optical disc of the present invention.

FIG. 4 is a bottom view showing the magnetic head device shown in FIG.3.

FIG. 5 is a bottom view showing another example of a magnetic headdevice adapted for the optical disc of the present invention.

FIG. 6 is an enlarged front view showing a magnetic head device adaptedfor the optical disc according to the present invention.

FIG. 7 is a front view showing still another example of a magnetic headdevice adapted for the optical disc according to the present invention.

FIG. 8 is a bottom view showing the magnetic head device shown in FIG.7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As a result of eager searches towards accomplishing the above object,the present inventors have arrived at an information that resistance tocorrosion of the optical disc having a thin metal film such as a rareearth-transition metal amorphous thin film can be significantly improvedby increasing the film thickness at an outer periphery of a protectivefilm.

The optical disc according to the present invention is proposed based onthe above information, and consists in an optical disc having afunctional film, formed by a thin metal film, and a protective film,characterized in that a maximum film thickness d at an outer peripheryof the protective film and a mean film thickness d₀ at a flat portion ona data area of the disc are related by the formula d≧2.0 d₀.

The optical disc of the present invention comprises a transparentsubstrate, a recording section including functional films such as arecording layer and a reflective layer formed on the substrate and aprotective layer formed on the recording section.

The transparent substrate may be formed of any substrate materialcommonly employed for an optical disc, such as a polycarbonate resin,polymethyl methacrylate resin or an amorphous polyolefin resin.

The functional film, such as a recording layer or a reflective layer,formed on the transparent substrate, may be arbitrarily selectedaccording to application and usage. In a digital audio disc or aso-called CD-ROM, a reflecting film of metal, such as Al, is depositedon a disc substrate on which a pattern of pits and lands have beentranscribed. In a magneto-optical disc, a perpendicular magnetic filmhaving magneto-optical properties, such as magnetic Kerr effect orFaraday effect, such as a rare earth-transition metal alloy amorphousfilm, exemplified by TbFeCo based amorphous thin film, is deposited as arecording layer on the transparent substrate.

The protective film is used for protecting the recording section fromimpacts or preventing corrosion or pitting of the recording section.

According to the present invention, in order that a highly corrosivemetal thin film, such as a rare earth-transition metal alloy amorphousfilm, used as the above-described functional film, may be positivelyprotected against corrosion by the protective film, such protective filmsatisfying the condition of d≧2.0 d₀, where d is a maximum filmthickness at the outer periphery and d₀ is a mean film thickness at aflat portion on a data area. That is, a protective film having a largerfilm thickness at the outer periphery, is employed.

While the protective film tends to permit moisture or O₂ as corrosionfactors to be intruded at, above all, an outer peripheral region of thedisc, it becomes possible to effectively prevent these corrosion factorsfrom being intruded by increasing the film thickness at the peripheralregion to provide improved water-proofing effects.

The protective film is formed by coating a UV-curable resin, such asacrylic UV-curable resins, by spin coating, followed by irradiation byUV rays.

For applying the UV curable resin by spin coating, a prescribed amountof the UV curable resin is applied dropwise at a mid part of therecording section, and the disc is set into rotation. The UV curableresin is propelled by a centrifugal force towards the outer rim of thedisc so as to be distributed over the entire recording zone. Whensubsequently the rotation of the disc is terminated, a coating film ofthe UV curable resin is formed on the recording zone with a heave at theouter rim of the disc. If the disc is allowed to stand, the UV curableresin at the heaved portion flows gradually towards the center toflatten out the outer rim of the disc. However, if the disc isirradiated with UV rays directly after termination of the disc rotationto cure the film with the resin being heaved at the rim, or if the discis rotated during irradiation of the UV rays to inhibit the flow ofresin from the heaved portion towards the center under the effects ofthe centrifugal force, the protective film may be formed with the heavedportion at the disc rim region.

A mean film thickness d₀ at the flat portion of the data area and themaximum film thickness d at the outer rim region may be controlled todesired values by controlling the number of revolutions of the disc, theduration of disc rotation for flinging the resin, the time which elapsessince the time of cessation of the disc rotation until the irradiationof UV rays or the number of rotations of the disc during irradiation ofthe disc with UV rays. The protective film satisfying the condition ofd≧2.0 d₀ may be formed by adjusting these conditions.

Meanwhile, the mean film thickness d₀ at the flat portion in the dataarea of the protective film of UV curable resin is desirably thicker infilm thickness for assuring protection of the recording zone. However,it is usually controlled to be 3 to 10 μm because an excessive thicknessof the flat portion tends to cause warping of the substrate due tocontraction on curing.

Meanwhile, the recording/reproducing apparatus for a magneto-opticaldisc on which a thin metal film having magneto-optical effects is formedas a functional film so as to be used as a recording layer is usuallyarranged so that a magnetic head device as a magnetic field generatingdevice is mounted facing the recording magnetic layer of the disc and anoptical pickup device for radiating a laser light beam is mounted facingthe side of the disc opposite to the recording magnetic layer with thesubstrate in-between. With such magnetic head device, only an extremelyweak magnetic field may be generated because of various constraints suchthat the magnetic head device needs to be positioned as close to themagneto-optical disc as possible.

For this reason, the magnetic head device made up of a magneticrecording zone and an actual sliding zone is employed. The magneticrecording zone has a magnetic head element for generating a recordingmagnetic field, whereas the actual sliding zone arranged as a more innerposition than the magnetic recording section, is contacted with themagneto-optical disc under a load imposed by a spring for supporting themagnetic recording zone at a small distance from the disc surface.

For recording the information by such magnetic head device, the actualsliding zone is kept in sliding contact with the protective film of themagneto-optical disc, so that the film thickness or shape of theprotective film of the magneto-optical disc and the shape of themagnetic head device need to be matched to one another.

When the functional film consisting of a metal thin film is aperpendicular recording magnetic film having a functional film formed bya thin metal film exhibiting magneto-optical effects, a difference L₁between a maximum film thickness d at the outer most region of aprotective film and a mean film thickness d₀ at a flat portion of a dataarea of the disc is 40 μm or less, or the protective film is graduallyincreased in thickness at an outer periphery of the disc to form acurved surface having a radius of curvature of 12.48 mm or more.

According to the present invention, recording is performed by a magnetichead device having a magnetic recording section and an actual slidingsection, and in that a height L₂ of the magnetic recording section fromthe sliding contact surface when the actual sliding section is slidinglycontacted with the sliding contact surface of the optical disc isrelated with a difference L₁ between a maximum film thickness d at theouter most region of a protective film and a mean film thickness d₀ at aflat portion of a data area of the disc by L₂ >L₁, or alternatively,when the magnetic recording section is positioned at an outer mostrecordable peripheral region of the optical disc, the actual slidingsection is slidingly contacted with the flat portion of the protectivefilm on the data area.

With the above-described magneto-optical disc, since too large adistance between the recording zone and the magnetic head element at anouter most part of the recording region of the magneto-optical disctends to produce recording failure due to spacing loss, the differenceL₁ between the maximum film thickness d at an outer periphery of thedisc and mean film thickness d₀ at the flat zone in the data area ispreferably not more than 40 μm. On the other hand, with the magnetichead device employing the magneto-optical disc, a height L₂ of themagnetic recording zone from the sliding surface of the magneto-opticaldisc is preferably related with L₁ by a formula L₂ >L₁. On the otherhand, with the above-described magneto-optical disc, a radius ofcurvature R₁ of a curved surface produced at the disc rim region of lessthan 12.48 mm is not desirable because the sliding zone of the magnetichead device is slid off from the curved surface to render it impossibleto record the information on the outer most region of the recording zoneof the magneto-optical disc. Besides, with the magnetic head deviceemploying the above-mentioned magneto-optical disc, if the magneticrecording region is positioned at an outer most position in therecording region of the disc, the sliding section is slidingly contactedwith the flat portion of the protective film on the data area, so thatthe distance between the magnetic head element and the recording sectionremains unchanged when recording at the outer most position of therecording region and hence the spacing loss is hardly produced.

Meanwhile, the protective film satisfying the above conditions may alsobe produced by controlling the number of revolutions of the disc forflinging the resin during coating the UV curable resin by spin coating,the duration of disc rotation for flinging the resin, the time whichelapses since the time of cessation of the disc rotation until theirradiation of UV rays or the number of rotation of the disc duringirradiation of the disc with UV rays.

With the optical disc according to the present invention, a protectivefilm is provided which satisfies the conditions that the maximum filmthickness d at the outer rim region and the mean film thickness d₀ atthe flat region on the data area are related to each other by d≧2.0 d₀.With the optical disc having such protective film, it is possible toprevent the moisture and oxygen from being intruded at the outer rimregion which was not possible with the conventional optical disc.Consequently, when a highly corrosive thin metal film, such as, forexample, rare earth-transition metal amorphous thin film, is employed asa functional film, the film exhibits superior corrosion resistance sothat recording/playback may be achieved with high reliability. Besides,with the optical disc according to the present invention, if thefunctional film formed by a metal thin film is a perpendicular recordingmagnetic film exhibiting magneto-optical effects, the difference L₁between the maximum film thickness d at the outer rim region of theprotective film and the mean film thickness d₀ at the flat portion inthe data area is 40 μm or less, or alternatively, the protective film isincreased gradually at an outer rim region to form a curved surfacehaving a radius of curvature R₁ of 12.48 mm or more, so that superiorresistance to corrosion is achieved and satisfactory recording may beachieved at the outer most position of the recording region to achieverecording and/or reproduction witch high reliability.

Besides, in accordance with the present invention, recording on theabove-described magneto-optical disc may be achieved with the magnetichead device having the magnetic recording section and the slidingsection, and the height L₂ of the magnetic recording section from thesliding surface when the sliding section is slidingly contacted with thesliding contact surface of the disc is related with the height L₁, whichis the difference between the maximum film thickness d at the outerperiphery of the protective film and the mean film thickness d₀ at theflat portion of the data area, by the formula L₂ >L₁, or alternatively,the sliding section is slidingly contacted with the flat region of theprotective film on the data area, so that satisfactory recording may beachieved at the outer most region of the recording section andrecording/playback may be achieved with high reliability and superiorcorrosion to resistance of the optical disc.

EXAMPLES

The present invention will be explained with reference to illustrativeExamples.

EXPERIMENTAL EXAMPLES

A first dielectric layer, a recording magnetic layer, a seconddielectric layer and a reflective layer, were deposited by sputteringstep by step on a polycarbonate substrate, without masking the outerregion, for forming a recording section. A UV-curable resin having aviscosity of 50 cps was applied to the recording section by spincoating. After cessation of the disc rotation, the disc was irradiatedwith UV rays within 1 second using a high-pressure mercury lamp, forforming a protective film, for producing magneto-optical discs (samplediscs 1 to 8).

The conditions under which the protective films were formed on the discsamples, namely the condition of flinging the resin during spin coatingand the number of revolutions of the disc during radiation of UV rays,the maximum film thickness d at the outer periphery of the protectivefilm and the mean film thickness d at the flat section of the data area,are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                                        film                                                                             thick-    film                                    condition of forming                                                                                     nessk-                                              protective film            do atss                                                           rpm during                                                                              of   outer                                                                uv rays                                                                              data                                                                               pe-                                         flinging                                                                               flinging                                                                               irradiation                                                                           area    riphery                                     (rpm)    time(sec)                                                                             (rpm)         (μm)                                                                       (μm)                                 ______________________________________                                        sample disc  1                                                                         2000     10       0       5.1  27.4                                  sample disc  2                                                                              2500                                                                                     8                 18.8                               sample disc  3                                                                              3000                                                                                     5                 13.2                               sample disc  4                                                                              3000                                                                                    10                 10.2                               sample disc  5                                                                              4000                                                                                     5                 11.1                               sample disc  6                                                                              3000                                                                                     5         100                                                                                   11.9                               sample disc  7                                                                              3000                                                                                     5         300                                                                                   12.8                               sample disc  8                                                                              3000                                                                                     5         500                                                                                   12.4                               ______________________________________                                    

A profile of a surface shape of the sample disc 1, as measured with P-1manufactured by TEN CALL Inc. is shown for reference in FIG. 1.

For comparison, magneto-optical discs (comparative discs 1 to 4) wereproduced in the same way as in Example 1 except applying a UV curableresin by spin coating and radiating UV rays after lapse of 5 secondssince cessation of the disc rotation for forming a protective film.

The time since cessation of disc rotation until irradiation of UV rayswas set to 5 seconds for adaptation to the time involved in handlingduring usual spin coating.

The conditions under which the protective films were formed on the discsamples, namely the condition of flinging the resin during spin coatingand the number of revolutions of the disc during radiation of UV rays,the maximum film thickness d at the outer periphery of the protectivefilm and the mean film thickness d₀ at the flat section of the dataarea, are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                                                        film                                                                             thick-  film                                      condition of forming                                                                                   thick-                                                                          ness                                               protective film     ness    do at                                                             rpm during                                                                              of   outer                                                                uv rays                                                                              data                                                                               pe-                                         flinging                                                                               flinging                                                                               irradiation                                                                           area    riphery                                     (rpm)    time(sec)                                                                             (rpm)         (μm)                                                                       (μm)                                 compar. disc 1                                                                         3000     5        0       4.9  5.6                                   compar. disc 2                                                                                2500                                                                                   5                  6.6                               compar. disc 3                                                                                2000                                                                                   5                  8.3                               compar. disc 4                                                                                1500                                                                                   5                 11.2                               ______________________________________                                    

A profile of a surface shape of the comparative discs is shown forreference in FIG. 2.

Ten each of the sample and comparative discs were allowed to stand for1000 hours under high temperature and high humidity conditions of 80° C.and 85% to check for the byte error rate before and after the discs wereallowed to stand. The byte error rate was checked by a drivermanufactured by SONY CORPORATION under the trade name of SMO-D501 up to1000th track from the outer most track. The results are shown in Table 3in which the amount of warping caused to the substrates of the discs isalso shown.

                  TABLE 3                                                         ______________________________________                                                byte error rate   wash of                                                     before allowing                                                                          after allowing                                                                           substrate                                               to stand                              (mrad)                          ______________________________________                                        sample disc 1                                                                           4.3 × 10.sup.-6                                                                      7.4 × 10.sup.-6                                                                    -0.4                                        sample disc 2                                                                             3.7 × 10.sup.-6                                                                    7.6 × 10.sup.-6                                                                    +0.8                                        sample disc 3                                                                             4.1 × 10.sup.-6                                                                    6.9 × 10.sup.-6                                                                    +0.2                                        sample disc 4                                                                             4.2 × 10.sup.-6                                                                    9.8 × 10.sup.-6                                                                    +0.6                                        sample disc 5                                                                             6.1 × 10.sup.-6                                                                    1.0 × 10.sup.-5                                                                    +0.5                                        sampie disc 6                                                                             4.8 × 10.sup.-6                                                                    8.7 × 10.sup.-6                                                                    +0.3                                        sample disc 7                                                                             3.9 × 10.sup.-6                                                                    9.1 × 10.sup.-6                                                                    +0.1                                        sample disc 8                                                                             5.3 × 10.sup.-6                                                                    8.9 × 10.sup.-6                                                                    ±0.0                                     compar. disc 1                                                                            5.7 × 10.sup.-6                                                                    4.1 × 10.sup.-5                                                                    +0.1                                        compar. disc 2                                                                            5.2 × 10.sup.-6                                                                    3.5 × 10.sup.-5                                                                    -1.2                                        compar. disc 3                                                                            4.4 × 10.sup.-6                                                                    2.3 × 10.sup.-5                                                                    -2.6                                        compar. disc 4                                                                            3.7 × 10.sup.-6                                                                    1.4 × 10.sup.-5                                                                    -4.8                                        ______________________________________                                    

It is seen from Table 3 that the sample discs 1 to 8 having heprotective films satisfying the condition of d≧2d₀ exhibit a low byteerror rate of not more than 1×10⁻⁵ even after left under the hightemperature high humidity conditions and hence are superior inresistance to corrosion. Besides, these sample discs undergo warping aslow as ±1 mrad or less and a satisfactory shape.

Conversely, the magneto-optical discs having the protective films notsatisfying the condition of d≧2d₀, that is the magneto-optical discshaving the protective films of uniform thicknesses on the entirerecording section, undergo a significant increase in byte error ratewhen left under the above-mentioned high temperature and high humidityconditions, if the protective films are of reduced thicknesses, whereas,if the protective films are of increased film thicknesses, thesubstrates undergo severe warping and are deteriorated in mechanicalproperties.

It is seen from the above results that the protective film satisfyingthe condition of d≧2d₀ in an optical disc is effective in providing anoptical disc having superior resistance to corrosion and a satisfactoryshape.

Constructional Example of Magnetic Head Device

Referring to the drawings, an illustrative example of a magnetic headdevice which may be employed with the optical disc of the presentinvention is explained. The recording/reproducing apparatus for amagneto-optical disc in which the functional film is a thin metal filmhaving magneto-optical effects is provided with the magnetic head deviceas the magnetic field generating device and the optical pickup deviceradiating the laser light for facing a recording magnetic layer of thedisc and facing the opposite side of the disc with the substratein-between. With such apparatus, the recording magnetic layer ispartially raised in temperature by the laser light radiated from theoptical pickup device and the recording magnetic field is applied by themagnetic head device for recording, while the laser light is applied bythe optical pickup device on the recording magnetic layer for playbackunder the magneto-optical effects. However, with the above-describedmagnetic head device, only an extremely small magnetic field may begenerated because of various constraints, such that it becomes necessaryto provide the magnetic head device as close to the magneto-optical discas possible.

Thus a magnetic head device made up of a magnetic recording section andan actual sliding section is employed. The magnetic head device is madeup of the magnetic recording section 1 having a magnetic head element,not shown, and the actual sliding section 2 supporting the recordingsection 1, as shown for example in FIG. 3. The sliding section 2 isprovided more inwards than the magnetic recording section 1 and iscontacted with a magneto-optical disc 3, as an optical disc of thepresent invention, under the bias of a spring, not shown, while themagnetic recording section is supported by the sliding section so as tobe separated by a height L₂ from a sliding surface 3a of themagneto-optical disc. For conformance of the magnetic head device to theoptical disc of the present invention, the height L₂ is related with adifference L₁ between the maximum film thickness d at the outerperiphery of the protective film of the magneto-optical disc and themean film thickness d₀ at the flat portion on the data area by arelation L₂ >L₁.

The above-described construction of the magnetic head device made up ofthe magnetic recording section and the sliding section is desirable forreducing the size and costs and for simplifying the construction.

FIG. 4 shows the magnetic head device in the direction of themagneto-optical disc 3. The magneto-optical disc 3 is rotated as shownby arrow M in FIG. 4 and the magnetic recording section 1 having themagnetic head element 5 and the actual sliding section 2 are providedforwardly and rearwardly along the sliding contact direction of themagnetic head device 4, respectively. For increasing the recordableregion of the magnetic recording section 1, the sliding section 2 isprovided more inwardly of the disc than the magnetic recording section 1for being extended along the sliding contact direction of the magnetichead device 4 with the magneto-optical disc 3. When the magneticrecording section of the magnetic head device records the information onthe outer most position of the recording region of the optical disc, thesliding section is slid in contact with the flat portion of theprotective film on the data area.

For conformance to the optical disc of the present invention having adiameter of 64 mm, an outer most recordable position is at a position of30.5 mm radius, the difference L₁ between the maximum film thickness dat the outer periphery and the mean film thickness d₀ at the flatportion of the data area is 40 μm, and a heaved portion is started at aposition of the radius of 31.0 mm at a radius of curvature R₁ of 12.48mm to form the heave at the outer most region, the sliding section isadapted to be positioned more inwardly than the radius 31.0 mm positionwhen the magnetic recording section records the information at theradius 30.5 mm position. That is, when a center 5a of a magnetic headelement 5 provided on the magnetic recording section 1 of the magnetichead device 4 is tracking the outer most recordable 30.5 mm radiusposition as indicated at r₁ of the magneto-optical disc 3, the slidingsection 2 is adapted to be slid in contact with the disc portion whichis situated more inwardly of the 31.0 mm radius heave start positionindicated at r₂, as shown in FIG. 4.

If, as shown in FIG. 6, the actual sliding section 12 is rounded incontour with an R of a radius of curvature R₂ so as to be free fromcontact with a curved portion 16 of the magneto-optical disc 13, and anactual sliding point 17 is provided on a disc sliding surface 12a of thesliding section 12, it becomes possible to approach the sliding contactpoint more closely to the center 15a of the magnetic head element 5 thanwhen the sliding section is not rounded, as indicated in FIG. 2. Thatis, by providing the actual sliding point 17 of the sliding section 12in the vicinity of an extension of a centerline 15a of the magnetic headelement 15 of the magnetic recording section 11, it becomes possible toelongate the actual sliding section 12 to improve disposition stabilityof the magnetic head device 14 in sliding contact with themagneto-optical disc 13 in a direction of arrow M in FIG. 4.

If the sliding contact position of the sliding site 17 is at the 31.0 mmradius position, the radius of curvature R₂ of the rounding of thesliding section 12 may be less than 12.48 mm (R₂ <12.48 mm ). However,if the sliding contact position r₅ of the actual sliding site 17 is moreinward than the 31.00 mm radius position, the radius of curvature R₂ maybe found as follows:

    (32.0-r.sub.5).sup.2 +(R.sub.2 -0.04).sup.2 =R.sub.2.sup.2

so that

    R.sub.2 =12.5r.sub.5.sup.2 800r.sub.5 +12800.02 (mm)

When the sliding contact position r₅ of the sliding contact point 17 ismore inward than the 31.0 mm radius position, contact with the curvedsurface 16 of the magneto-optical disc may be inhibited by reducing theradius of curvature R₂ of the sliding section 12 so as to be smallerthan the value obtained by the above formula.

Although the outer and inner sides of the sliding section 12 are roundedin FIG. 6, only the outer side of the sliding section 12 may be roundedfor achieving comparable results.

As an alternative of the shape of the magnetic head device forconformance to the optical disc of the present invention, an actualsliding section 22 may be provided at a portion of the radially innerzone of the magnetic recording section 21 of a magnetic recording device24, as shown in FIG. 7. The actual sliding section 22 has a surfacefacing the disc 22a which is rounded so as to be free from contact withthe curved surface of the magneto-optical disc 23 to form an actualsliding point 25. With the present magnetic head device 24, the heightL₂ of the magnetic recording section 21 from the sliding contact surface23a of the magneto-optical disc 23 is related with the difference L₁between the maximum film thickness d at the outer periphery of theprotective film of the magneto-optical disc and the mean film thicknessd₀ at the flat portion of the data area by L₂ >L₁, as shown in FIG. 7.

FIG. 8 shows the magnetic head device as viewed in the direction ofmagneto-optical disc. When a center 26a of a magnetic head element 26provided on the magnetic recording section 21 of the magnetic headdevice 24 is tracking the recordable outer most 30.5 mm radius positionas indicated at r₆ of the magneto-optical disc 23, the actual slidingpoint 7 is adapted for being slidingly contacted with a positionradially inward of the 31.0 mm radius heave start position r₇, as shownin FIG. 8.

It is seen from above that the present invention provides an opticaldisc having a protective film satisfying the condition of d≧2.0 d₀,where d is the maximum film thickness at the outer most region and d₀the mean film thickness at the flat portion of the data area, whereby itis possible to prevent corrosion or pitting of a highly corrosive thinmetal film as a functional film to achieve superior corrosionresistance. Consequently, a practically useful service life may beachieved with the magneto-optical disc to permit diversification of therecording system of the optical disc.

Besides, with the optical disc according to the present invention, ifthe functional film formed by a metal thin film is a perpendicularrecording magnetic film exhibiting magneto-optical effects, thedifference L₁ between the maximum film thickness d at the outer rimregion of the protective film and the mean film thickness d₀ at the flatportion in the data area is 40 μm or less, or alternatively, theprotective film is increased gradually at an outer rim region to form acurved surface having a radius of curvature R₁ of 12.48 mm or more, sothat superior resistance to corrosion is achieved and satisfactoryrecording may be achieved at the outer most position of the recordingregion to achieve recording and/or reproduction with highreliability-and an optical disc with superior quality.

In addition, in accordance with the present invention, recording on theabove-described magneto-optical disc may be achieved with the magnetichead device having the magnetic recording section and the slidingsection, and the height L₂ of the magnetic recording section from thesliding surface when the sliding section is slidingly contacted with thesliding contact surface of the disc is related with the height L₁, whichis the difference between the maximum film thickness d at the outerperiphery of the protective film and the mean film thickness d₀ at theflat portion of the data area, by the formula L₂ >L₁, or alternatively,the sliding section is slidingly contacted with the flat region of theprotective film on the data area, when the magnetic recording section ispositioned at an outer most position in the recordable region of theoptical disc so that satisfactory recording may be achieved at the outermost region of the recording section and recording/playback may beachieved with high reliability, while superior corrosion to resistanceof the optical disc may be realized to provide an optical disc withsuperior quality.

What is claimed is:
 1. A method of reducing a byte error rate of datareproduced from an optical disc arising from corrosion of a functionallayer of the optical disc, comprising the steps of:providing asubstrate; arranging a functional film over the substrate; and forming aprotective film over the functional film, wherein the protective filmhas a maximum film thickness d at an outer periphery of the protectivefilm and a mean film thickness d₀ at a flat portion of the protectivefilm on a data area of the disc which are related by

    2d.sub.0 ≦d-d.sub.0 +40 mm,

and wherein a radius of curvature of the protective film at the outerperiphery of the film is equal to or greater than a radius of curvatureof a portion of a sliding contact surface of a magnetic head deviceradially extending toward said outer periphery of said disc from thereading portion of the magnetic head device, and wherein the radius ofcurvature of said protective film at said outer periphery is equal to orgreater than 12.48 mm.
 2. The method of claim 1, further comprising thestep of recording data on the functional layer when a section of theouter portion of the sliding contact surface overlaps a portion of theprotective film between the flat portion and an outer edge of the disk.3. The method of claim 1, further comprising the step of reading datastored at an outer most recordable radius position of the optical diskusing a magnetic head device with an elongated sliding contact surface.4. The method of claim 3, further comprising the step of positioning amagnetic head element of the magnetic head device substantially over asliding contact point when said reading step is being performed.
 5. Themethod of claim 1, wherein the maximum film thickness d of theprotective film at the outer periphery and the mean film thickness d₀ ofthe protective film at the flat portion are related by

    2d.sub.0 ≦d≦d.sub.0 +40μm.


6. A method of reducing a byte error rate of data reproduced from anoptical disc arising from corrosion of a functional layer of the opticaldisc, comprising the steps of:providing a substrate; arranging afunctional film over the substrate; and forming a protective film overthe functional film, wherein the protective film has a maximum filmthickness d at an outer periphery of the protective film and a mean filmthickness d₀ at a flat portion of the protective film on a data area ofthe disc which are related by

    2d.sub.0 ≦d≦d.sub.0 +40 mm,

and wherein a space over a curved portion of the protective film betweenthe flat portion and the outer edge of the disk is occupied by a firstsection of a sliding contact surface of a magnetic head device when asecond section of the sliding contact surface contacts the flat portion.7. The method of claim 6, wherein the maximum film thickness d of theprotective film at the outer periphery and the mean film thickness d₀ ofthe protective film at the flat portion are related by

    2d.sub.0 ≦d≦d.sub.0 +40μm.


8. 8. A method of reducing a byte error rate of data reproduced from anoptical disc arising from corrosion of a functional layer of the opticaldisc, comprising the steps of:providing a substrate; arranging afunctional film over the substrate; and forming a protective film overthe functional film, wherein the protective film has a maximum filmthickness d at an outer periphery of the protective film and a mean filmthickness d₀ at a flat portion of the protective film on a data area ofthe disc which are related by

    2d.sub.0 ≦d<d.sub.0 +40 mm,

and wherein a space over a curved portion of the protective film betweenthe flat portion and the outer edge of the disk is occupied by a sectionof a sliding contact surface of a magnetic head device, said section ofsaid sliding contact surface having a radius of curvature less than theradius of curvature of the curved portion of the protective film, whenan outer most recordable area of the functional film is magneticallycommunicating with a magnetic recording section of the magnetic headdevice.